1. Field of the Invention
The present invention relates to a structure of a guide bush mounted on a column of an automatic lathe to support a workpiece for rotation and sliding, and a method of forming a diamond-like carbon film over the inner circumference of the guide bush.
2. Description of the Related Art
A guide bush is a component mounted on a column of an automatic lathe to support a workpiece for rotation and sliding.
Further explaining about the guide bush in detail, it is in an approximate cylindrical form having a center bore formed along its center axis, including a taper outer surface formed on one longitudinal end portion and provided with slits to make the end portion elastic, a threaded portion formed on another end portion to be mounted on a column of an automatic lathe, and an inner surface for holding a workpiece inside a portion in which the taper outer surface is formed, mounted on an automatic lathe so as to hold a workpiece inserted into the center bore with the inner surface so that the workpiece can rotate and axially slide at a portion near a cutting tool.
Therefore, the inner surface of the guide bush always contacts and rotates with a workpiece, and a workpiece rotates and axially slides on the inner surface. Thus, the inner surface of the guide bush supporting a workpiece can be abraded easily.
So, a guide bush is proposed in JP-A No. 4-141303 which has an inner surface, which is in contact with a workpiece because of rotation and sliding, coated with a cemented carbide alloy or a ceramic.
When the inner circumference of a guide bush is provided with a cemented carbide alloy or a ceramic excellent in abrasion resistance and heat resistance, the abrasion of the inner surface of the guide bush can be reduced to some extent.
However, even when the guide bush is used on an automatic lathe, there was problems because of a large coefficient of friction and a low thermal conductivity of the cemented carbide alloy and the ceramic, that the workpiece is damaged or seizing occurs due to decrease in the diametrical clearance between the guide bush and the workpiece in case of heavy machining in which the depth of cut is large and the cutting speed is high.
Thus, it is proposed that the cemented carbide alloy provided on the inner surface of the guide bush is covered with a diamond-like carbon film to reduce abrasion of the inner surface.
The diamond-like carbon film is a black coating, and is a hard carbon film having properties close to those of diamond. The diamond-like carbon film has excellent properties of high mechanical hardness, a low coefficient of friction, good electric insulation, and high corrosion-resistance.
So, it enables the guide bush to improve abrasion resistance dramatically and to avoid seizing that the surface of the cemented carbide alloy on the inner surface of the guide bush is covered with the diamond-like carbon film.
Incidentally, the diamond-like carbon film is a hydrogenated amorphus carbon film and generally called diamond-like carbon film (abbreviated as "DLC film") because of above-described diamond-like properties, and also called i-carbon film.
DLC Film Forming Method on a Guide Bush According to a Conventional Art: FIG. 16
A method of forming DLC film over an inner surface of a guide bush, according to the conventional art will be described referring to FIG. 16 hereinafter.
As shown in FIG. 16, a guide bush 11 on which DLC film will be formed is mounted in a vacuum vessel 61 which comprises of a gas inlet port 63 and a gas outlet port 65. The guide bush 11 is quenched and tempered after an outer form and an inner form thereof are made up with an alloy tool steel (SK steel). And a cemented carbide alloy is provided on the inner surface 11b of the guide bush 11.
A DC voltage is applied to the guide bush 11 by a DC power source 73, a positive DC voltage is applied to the anode 79 disposed in the vacuum vessel 61 opposite the guide bush 11 by an anode power source 75, and an AC voltage is applied to the filament 81 by a filament power source 77.
Further, the vacuum vessel 61 is evacuated by an evacuating means which is not shown through the gas outlet port 65, and gas containing carbon is introduced through the gas inlet port 63. Thus, plasma is produced in the vacuum vessel 61 to form DLC film on the surface of the guide bush 11 containing the inner surface 11b thereof.
In the case of the method of forming DLC film with the unit shown in FIG. 16, plasma caused by the DC voltage applied to the guide bush 11, and plasma caused by the filament 81 to which the AC voltage is applied and the anode 79 to which the DC voltage is applied, is produced.
Then, according to the pressure in the vacuum vessel 61 at the time the DLC film is formed, either the plasma around the guide bush 11 or the plasma beside the filament 81 and the anode 79 becomes a main source of forming DLC film.
In the case of the above-described conventional DLC film forming method, the plasma which is produced in a portion around the guide bush 11 becomes a main source of decomposing the gas containing carbon and forming DLC film when the pressure in the vacuum vessel 61 is above 3.times.10.sup.-3 torr.
Then, it is possible to form DLC film on the outer surface of the guide bush 11, but the DLC film formed on the inner surface of the center bore 11j is poor at adhesion, and properties thereof, hardness or the like, are also poor.
This is caused by that the same electrical potential is applied to the guide bush 11, the inside of the center bore 11j becomes a space where electrodes having the same potential are opposite to each other, and the plasma in the center bore 11j causes abnormal discharge called hollow discharge.
The DLC film formed by this hollow discharge is polymer-like and poor at adhesion. Thus it flakes off easily from the inner surface 11b of the guide bush 11, and the hardness thereof is low.
On the other hand, the plasma produced beside the filament 81 and the anode 79 contributes to the forming of DLC film more than the plasma around the guide bush 11 when the pressure in the vacuum vessel 61 is below 3.times.10.sup.-3 torr.
In this case, it is possible to form DLC film uniformly on the outer surface of the guide bush 11, but it is not possible in the direction of the length on the inner surface of the center bore 11j of the guide bush 11.
Here, carbon ions ionized by the plasma produced beside the filament 81 and the anode 79 pile up pulled by the negative DC potential applied to the guide bush 11, and form DLC film.
DLC film is formed by chemical vapor deposition process when the above-described pressure in the vacuum vessel 61 is above 3.times.10.sup.-3 torr. But it is formed by physical vapor deposition process when the pressure is below 3.times.10.sup.3 torr.
Thus, when DLC film is formed under the contribution of the plasma produced beside the filament 81 and the anode 79, the thickness of the DCL film on the inner surface of the center bore 11j of the guide bush 11 decreases with the distance from the open end.
As a result, it is impossible to form DLC film in uniform thickness in the direction of the length of the guide bush 11 on the inner surface 11b thereof.
Further, in the case of the guide bush on which the above-described conventional DLC film is formed, a cemented carbide alloy made of alloy tool steel (SK steel) is provided on the inner surface of the guide bush base, and DLC film is formed on the surface of the cemented carbide alloy. G-2 (Japanese Industrial Standards: JIS) is frequently used as the cemented carbide alloy.
Generally used chemical composition of the G-2 (JIS) is 87-90% of tungsten (W), 5-7% of carbon (C) and 5-7% of cobalt (Co) as a binder.
When DLC film is directly formed on the cemented carbide alloy of this composition, the DLC film can not be formed with good adhesion, and the problem that it easily peels off occurs.